The air pressure within most pneumatic tires is controlled by introducing or evacuating compressed air through a valve located on the tire rim. These valves are located in a valve stem extending outward from the rim and tire. The valve is operated by pressing downward on the valve core which allows compressed air to be introduced into the tire. In order to prevent the valve from becoming contaminated with debris, a valve cap is removably mounted to the valve stem.
Although the valve cap must be removable for proper maintenance of the air pressure within the tire, this creates several problems. First, vandals can easily remove the valve cap, gain access to the valve, and release all the air from the tire. Second, valve caps are easily stolen, requiring the tire owner to purchase new valve caps.
In an effort to solve these problems, U.S. Pat. No. 5,630,687 ("'687") teaches a tamper resistant valve cap having an inner liner adapted for threaded engagement with a pneumatic tire valve stem, an outer sleeve rotatably mounted about the inner liner, and a screw inserted through the outer sleeve and into an annular channel surrounding the top of the inner liner. Depending on the positioning of the screw, the outer sleeve and inner liner can either move as a single unit, or the sleeve can freely rotate about the liner. When the sleeve and liner move as a unit, the liner can be removed from the valve stem. When the sleeve freely rotates about the liner, it is difficult or impossible for the liner to be removed from the stem.
Although this device does hinder unauthorized removal of a valve cap, it creates some problems of its own. Most notably, the screw means for securing the outer sleeve to the inner liner makes the device difficult to use and maintain. The screw is quite small, and, therefore, difficult to manipulate. This situation is made even worse if the valve cap is mounted on the tire in such a way that the screw is in a particularly inaccessible location. Because of its size, the screw is also susceptible to unintended loss. The vibration of the tire may also result in the dislodging of the screw, and, consequently, the loss of the sleeve.